In semiconductor manufacture, micro lithography is used in the formation of integrated circuits on a semiconductor wafer. During a lithographic process, a form of radiant energy such as ultraviolet light, is passed through a mask or reticle and onto the semiconductor wafer. The mask contains opaque and transparent areas or regions formed in a predetermined pattern. A grating pattern, for example, may be used to define parallel spaced conducting lines on a semiconductor wafer. The ultraviolet light exposes the mask pattern on a layer of resist formed on the wafer. The resist is then developed for removing either the exposed portions of resist for a positive resist or the unexposed portions of resist for a negative resist. The patterned resist can then be used during a subsequent semiconductor fabrication process such as ion implantation or etching.
FIG. 1 illustrates a complex photoresist pattern 10 that has been formed on a portion of a semiconductor wafer using an optical lithographic technique. In FIG. 1, areas on the wafer that are covered with photoresist are shaded with diagonal dashed lines. The photoresist pattern 10 formed on the wafer includes a repetitive arrangement of generally T-shaped sections 12A-D shaped substantially as shown. The T-shaped sections 12A-D are arranged in mirror image pairs. In addition, a pair of parallel spaced borders 14 are situated generally perpendicular to the T-shaped sections 12A-D and frame the lower portion of the pattern 10. Another pair of parallel spaced borders 14A, formed as mirror images of borders 14, frame the upper portion of the pattern 10. The borders have an irregular peripheral configurations substantially as shown.
If a positive photoresist is used, the photoresist which forms the T-shaped sections 12A-D and borders 14, 14A would correspond to opaque regions on the mask. The remaining areas of the wafer have no photoresist and would correspond to transparent or light transmissive areas on the mask. Light passing through these transparent areas of the mask pattern during the photolithographic process functions to develop the positive photoresist which is then removed. Conversely for a negative tone photoresist, the areas of the wafer having photoresist (T-shaped sections 12A-D, borders 14, 14A) would correspond to the transparent areas of the mask.
With reference to FIG. 2, a mask pattern 16 suitable for developing a negative tone resist into the photoresist pattern 10 illustrated in FIG. 1 is shown. In FIG. 2, the opaque areas of the mask pattern 16 have diagonal solid lines and transparent areas are clear. The mask pattern 16 includes transparent areas formed with mirror image pairs of T-shaped sections 18A-D, and parallel spaced borders 20, 20A shaped substantially as shown. As before, the borders 20, 20A frame the T-shaped sections 18A-D and are perpendicular to the T-shaped sections 18A-D. The transparent areas of the mask pattern 16 are used for developing the negative tone photoresist that remains in the desired pattern 10 (FIG. 1) on the wafer.
Such a conventional mask arrangement works well for forming semiconductor structures having feature sizes that are larger than about 0.5.mu.. As microcircuit densities have increased, however, the size of the features of semiconductor devices, such as those represented by the photoresist pattern 10 of FIG. 1, have decreased to the sub micron level. These sub micron features may include the width and spacing of metal conducting lines or the size of various geometric features of active semiconductor devices. The requirement of sub micron features in semiconductor manufacture has necessitated the development of improved lithographic processes and systems. One such improved lithographic process is known as phase shift lithography.
With phase shift lithography the interference of light rays is used to overcome diffraction and improve the resolution and depth of optical images projected onto a target. In phase shift lithography, the phase of an exposure light at the object is controlled such that adjacent bright areas are formed preferably 180 degrees out of phase with one another. Dark regions are thus produced between the bright areas by destructive interference even when diffraction would otherwise cause these areas to be lit. This technique improves total resolution at the object (i.e. wafer) and allows resolutions as fine as 0.25 .mu.m to occur.
Whereas a conventional lithographic mask contains only transparent and opaque areas, a phase shifting mask is constructed with a repetitive pattern formed of three distinct areas or layers of material. An opaque layer provides areas that allow no light transmission, a light transmission layer provides areas which allow close to 100% of light to pass through and a phase shift layer provides areas which allow close to 100% of light to pass through but phase shifted 180 degrees (.pi.) from the light passing through the light transmissive areas. The light transmissive areas and phase shift areas are situated such that light rays diffracted from the edges of the opaque layer and through the light transmissive and phase shift areas is canceled out in a darkened area there between. This creates a pattern of dark and bright areas which can be used to clearly delineate features of a pattern defined by the mask on a photopatterned semiconductor wafer.
Recently, different techniques have been developed in the art for fabricating different types of phase shifting masks. One type of phase shifting mask, named after a pioneer researcher in the field, M. D. Levenson, is known as a "Levenson" or "alternating aperture" phase shift mask. Such a mask is typically formed on a transparent substrate such as polished quartz. An opaque layer, formed of a material such as chromium, is deposited on the transparent substrate and etched with a pattern of apertures. This forms opaque areas on the mask which combined with the pattern of apertures carry the desired pattern. With a phase shifting mask the transparent areas and phase shifting areas are formed within the apertures in an alternating pattern with respect to the opaque areas.
The phase shift areas of the mask pattern may be formed by depositing a phase shifting material into every other aperture (i.e. additive process). Alternately, phase shift areas may be formed by etching a groove in every other aperture (i.e. subtractive process). With this type of phase shift structure the light passing through a grooved aperture travels a shorter distance in the substrate relative to light passing through an adjacent aperture formed over the full thickness of the substrate. Light beams exiting adjacent apertures of the mask therefore have a phase difference. This phase difference is preferably 180.degree. (.pi.), or whole multiple thereof, so that the light waves cancel out at the wafer. The thicknesses of the substrate for the phase shift areas and light transmission areas of a mask pattern can be calculated by the formula: EQU t=i.lambda./2(n-1)
where
t=thickness PA1 i=an odd integer PA1 .lambda.=wavelength of exposure light PA1 n=refractive index of substrate at the exposure wavelength
With reference to FIG. 2A, a prior art phase shifting mask pattern 22 for a Levenson (alternating aperture) phase shifting mask is shown. The phase shifting mask may be formed on a transparent substrate (e.g., quartz) having an opaque material (e.g. chromium) deposited thereon. Opaque areas of the mask pattern are represented by the diagonal shading, light transmissive areas are clear and phase shift areas have vertical shading.
As with the previous mask pattern 16 (FIG. 2), the phase shift mask pattern 22 (FIG. 2A) includes transparent areas formed with mirror image pairs of T-shaped sections 24A-D and parallel spaced borders 26, 26A. As represented by the vertically shaded areas, every other T-shaped section 24B, 24D and every other border 26, 26A is formed as a phase shift area. The phase shift areas alternate with light transmission areas wherein no phase shift occurs (i.e., alternating apertures). Phase shifting may be accomplished by forming the phase shift areas (or alternately the light transmission areas) as grooves in the substrate to a predetermined depth. Alternately a phase shift material may be deposited on the substrate to form the phase shift areas.
Because of their complexity, phase shifting mask patterns are often generated using automated computer aided design techniques (Auto-CAD). As an example, the technical article entitled "Investigating Phase-Shifting Mask Layout Issues Using a CAD Toolkit" by Wong et al., International Electron Devices Meeting, Washington, D.C., Dec. 8-11, 1991, described a CAD design process for phase shift masks.
One problem with a phase shifting mask constructed with such a complex mask pattern 22 is that there are numerous phase conflict areas which cause the projected image to become degraded. In general, a phase conflict occurs where two areas of the same phase occur together on the mask pattern 22 in very close proximity. Two of these phase conflict areas for adjacent 0.degree. areas are designated as 28 and 30. Two of these phase conflict areas for adjacent 180.degree. areas are designated 28A and 30A.
The opaque material in the phase conflict areas, such as opaque sections 32, 34, 32A, 34A may have a relatively narrow width which is below the resolution limit for the system. These narrow width opaque sections (32, 34, 32A, 34A) will therefore not "resolve" and the feature represented by the opaque material will not print clearly on the wafer. For this reason, phase shifting lithography may not provide satisfactory results for many complex patterns used in semiconductor manufacture.
In view of this and other problems, there is a need in the art for improved phase shifting masks suitable for forming complex patterns. Accordingly, it is an object of the present invention to provide an improved method of making phase shifting masks for photolithography. It is a further object of the present invention to provide an improved phase shifting mask in which the resolution of projected features in phase conflict areas is improved. It is a still further object of the present invention to provide an improved method for making phase shifting masks which is adaptable to large scale semiconductor manufacture and which is compatible with Auto-Cad mask layout techniques.